1. Field of the Invention
The invention relates in general to spectroscopy instruments for remote ultraviolet to infrared spectrum analysis and chemical analysis. More particularly, the invention relates to portable acousto-optical based spectrophotometers (spectrometers) for spectrum analysis, chemical analysis and of the species of interest.
2. Description of the Prior Art
Spectrometers are widely used for many applications such as chemical analysis, remote sensing, quality control, environmental monitoring, spaceborne measurements, and optical measurements. Most spectrometers are based on using dispersive elements such as prisms, gratings or etalons. These spectrometers typically have moving parts, which induce spurious readings due to vibrations, and have limited use due to their limited spectral range and resolution capabilities, which in turn limits their use when needed for portable field applications.
Acousto-Optical tunable filters (AOTF)s are very powerful tools that can be used in many spectroscopic applications, including absorption, emission, fluorescence, Raman, and laser-induced breakdown spectroscopy (LIBS) measurement instruments and inside of traditional, or fiber laser cavity for choosing and tuning frequency of light radiation. They are lightweight, compact, and very useful for field-portable applications. They have no moving parts, are all solid-state construction, require low power, and are insensitive to vibrations. They have high spectral resolution, large field of view and high throughput. They offer high-speed tuning and scanning of wavelengths and have reliable and reproducible operation under computer control. The wavelength tuning can be random or sequential, and an AOTF can be operated at multiple frequencies. AOTFs can be used for generating an arbitrary spectral response and can be used for polarization-sensitive applications. Such filters can be used as a replacement for filter wheels and gratings. Laser wavelength tuning applications use AOTF both inside and outside the laser cavity. AOTF spectrometers have been designed with both high sensitivity and high resolution. AOTF instruments are used for detection of chemical and biological agents and for medical and pharmaceutical applications, as well as for environmental sensing applications.
The AOTF is a birefringent crystal having an acoustic transducer bonded to one face. Broad-band light radiation passing through a crystal can be diffracted into specific wavelengths by application of a radio-frequency (rf) driving signal to the crystal transducer. The AOTF device can be used as a part of a spectrometer or other optical instrument. The AOTF has several advantages over alternative spectroscopic techniques such as filter wheels, diffraction gratings, and Fourier transform infrared. The AOTF crystal cell is small, fast, and reliable and has no moving parts. Because of these features, the AOTF crystal cell can be used for applications such as chemical process control, medical diagnostics, spectral radiometry, and real time composition analysis in the production environment. When coupled to fiber optics, the AOTF can be located remote from the sample of interest. This remote location has an advantage when the sample is in a harsh or potentially explosive environment. Fiber coupling reduces a chance of explosion by eliminating all electrical voltage in the sampling region. Among the attractive features of AOTFs are their small size, light-weight, computer-controlled operation, large wavelength tuning range, and reasonably high spectral resolution. Additionally, their operation can be made ultra-sensitive by using advanced signal-processing algorithm.
The AO effect allowed for the development of an AOTF, which was not discovered until much later in 1967, when a new type of AO interaction was discovered in anisotropic crystals (in such crystals the speed of light with different vector of polarization is different). Two years later this AO effect was used in a collinear AOTF cell using lithium niobate (LiNbO3) and successfully demonstrated. In a collinear AOTF, the incident light, the acoustic wave, and the diffracted beam all travel in the same direction. A number of different crystals, i.e., quartz, LiNbO3, etc., allow collinear diffraction of light with either longitudinal or shear acoustic wave propagation. Chang generalized the design of an AOTF cell by introducing the concept of a noncollinear AOTF using tellurium dioxide (TeO2), a birefringent crystal (a crystal having two refractive indices) that cannot exhibit collinear interaction because of its crystal symmetry. In a noncollinear AOTF cell the incident light, the diffracted light, and the acoustic wave do not travel in the same direction. At present, a number of AOTF cells and AOS's are available commercially and as research instruments. The United States, Russia, and Japan are the leading players in this technology. Since the first commercial offering of AOTF by the Isomat Corporation in 1975, this technology has made much progress. Near-IR (900-1800 nm) AOS are now commercially available.
An AOTF is essentially a real-time programmable filter whose operation can be described as follows. When white light is incident on the filter, it passes only a selected number of narrow bands corresponding to the applied rf-signals. The filter can be used to pass light with either a single wavelength or multiple wavelengths, depending upon the number of applied rf-signals. Either a collinear or a non-collinear geometry can be used in designing an AOTF cell, based on the symmetry properties of the anisotropic crystal under consideration. The incident light is linearly polarized by a polarizer in front of the crystal before it enters the AOTF cell. As this polarized light passes through the cell, it is diffracted in the same direction by a diffraction grating set up by the collinearly traveling sound wave. Owing to conservation of energy, the frequency of the diffracted light is Doppler shifted, but this frequency shift is insignificant and can be ignored. Based on conservation of momentum, a tuning relationship can establish between the center wavelength of the filter and the applied rf-signal. Many excellent review articles on AOTF technology and applications are available, for example see Gottlieb, M. S., “Acousto-optic tunable filter,” Design and Fabrication of Acousto-Optic Devices, A. P. Goutzoulis and D. R. Pape, eds., Marcel Dekker, New York, 1994, pp. 197-283; Gupta, N., ed., Proceedings of the First Army Research Laboratory Acousto-Optic Tunable Filter Workshop, Army Research Laboratory, ARL-SR-54 (1997); and Gupta, N. and Fell, N. F., Jr., “A compact collinear Raman spectrometer,” Talanta 45, 279-284 (1997).
An example of a spectrometer using AO crystal cells includes U.S. Pat. No. 5,120,961 entitled “High sensitivity acousto-optic tunable filter spectrometer,” which teaches of using an acousto-optical filter (AOTF) device in a spectrometer. This spectrometer operates by using continuous wave RF-excitation through the crystal, wherein the spectrometer provides control and modulation of the RF-source. Noise is minimized by a lock-in amplifier that demodulates the modulation frequency. Fiber optics are used to connect the crystal to the source, and the source to the detection system. In contrast, the present invention preferably uses pulsed-wave RF-excitation through the crystal(s) for control of the AO crystal cell.
An ongoing problem in using spectrometers in more applications is their miniaturization. The size of a spectrometer is limited by their required precision and accuracy of measurements because of existing relationships between optical spectral resolution, spectral range of a spectrometer and its inherent physical dimensions. The optical spectral resolution of commonly manufactured spectrometers is proportional to their dimensions. This is a noted and important limitation for miniaturization of spectrometers, which heretofore generally cannot be circumvented. Unfortunately, since precise spectrometers for use in environmental analysis are often bulky, costly, and expensive to transport and install, many known and important applications of spectrometers remain unimplemented due to cost and/or inconvenience.
In particular, miniaturization of equipment for Raman spectroscopy applications is a current problem. Raman spectroscopy is a powerful analytical technique that provides complete identification of chemical agents based on their electronic vibrational energy levels. This technique is typically confined to controlled laboratory environments because Raman signals are very small and require very high sensitivity, high-resolution spectrometers with special attachments to do these measurements. Also, highly trained personnel are needed to setup and run these experiments. Fluorescence measurements are used to detect biological particles. AO-type spectrometers used in Raman spectroscopic measurements, generally require high sensitivity and resolution. Previously known such spectrometers include a collinear acousto-optic spectrometer called a “Quartz 4,” which was constructed in the former Soviet Union and made of quartz. This instrument provided measurements in the visible light spectral range of 430-800 nm and less sensitive compared to the instant inventions AO spectrometer subassembly provided herein due to the crystal design. Additionally, known Raman and fluorescence spectroscopic systems are generally large, cumbersome, hard to maintain, take much time to setup, and operate over limited spectral ranges. For applications that require field portability, these systems cannot generally be used. Also, most currently used spectroscopic systems cannot be used to take both Raman and fluorescence spectral measurements simultaneously because each of these measurement techniques has different system requirements. Most of these current systems have moving parts such as gratings, filter wheels, or moving mirrors for tuning at a desired optical wavelength.
Thus, there is a need for a portable AOTF spectrometer system that is relatively less expensive compared to currently available systems that can be produced and packaged for field hand-held use by non-experts. Thus, the present invention addresses these problems by providing an autonomous, integrated spectrum-measurement-based spectroscopy system for UV-Vis-IR spectral ranges of interest, and a system that can be adapted for Raman and fluorescence spectral measurements as well.